1. Field of the Invention
This invention relates to a method for the production of a silicon nitride material with improved properties obtained by forming a phase of closely packed crystals in the surface region thereof. More particularly, the present invention relates to a method for the production of silicon nitride material excelling in resistance to heat, resistance to thermal shock, and mechanical strength, which method comprises giving a silicon nitride substrate a specific surface treatment thereby educing in the surface region of the substrate a phase of closely packed crystals of silicon nitride.
2. Prior Art Statement
Since sintered articles of silicon nitride are excellent in physical properties such as resistance to heat, resistance to thermal shock, and mechanical strength and are capable of being molded in desired shapes with high accuracy, efforts have been made to promote their utilization as heat-resistant structural materials in gas turbines and automobile engines. These sintered articles have bright prospects for expanding application.
Unfortunately, silicon nitride has a serious disadvantage in that it is difficult to sinter and the sintered articles of this compound are susceptible to oxidation in the air at elevated temperatures. Various methods aimed at eliminating this disadvantage have been proposed. These include, for example, a reaction sintering method which comprises mixing silicon nitride powder with a specific oxide powder and heating the resultant mixture at an elevated temperature thereby inducing mutual reaction of the two compounds, a hot-press method which comprises mixing silicon nitride powder with various oxide powders serving as a catalyst and treating the resultant mixture at elevated temperatures under increased pressure, and a high-pressure gas method which effects the reaction in an atmosphere of nitrogen gas kept at elevated temperatures. However, these methods require use of special devices and entail large consumption of energy. Moreover, the sintered articles produced by these methods do not always acquire a dense texture and more often than not contain pores. For example, even in the case of a sintered article of silicon nitride obtained by the hot-press treatment at 1,850.degree. C. for 1 hour, an observation of a cross section of this sintered article reveals that numerous pores about 2 to 3 .mu.m in diameter are present in the texture.
If a sintered article contains such pores, it is liable to undergo oxidation in the air at elevated temperatures and induce various detrimental phenomena. For example, the oxygen in the air diffuses through the pores into the texture of the sintered article and, at a temperature of about 700.degree. C., forms a thin film amorphous silicon dioxide texture and, then at a higher temperature of about 1,200.degree. C., crystallizes as cristobalite and, in the meantime, gives rise in the surface region of the sintered article to an oxidation product of large bulk and low melting point comprising silicate containing a sintering aid like Y.sub.2 O.sub.3 and a vitreous matter. Since the thermal expansion coefficient of the oxidation product differs considerably from that of silicon nitride, the oxidation product eventually imparts cracks and separations to the aforementioned sintered article of silicon nitride.
When a silicon nitride material lacks dense texture and contains pores as described above, it offers poor resistance to oxidation at elevated temperatures and, therefore, has only limited utility as structural materials, which by nature require resistance to high temperature. As means for modifying the surface of the silicon nitride material, a ceramic coating method, a vacuum deposition method using plasma flame spraying or CVD, and a method for forming a protective coating film on the surface of a substrate as by an ion beam or laser are now under a study.
No method has yet been developed which is capable of uniformly and economically superposing on the surface of a silicon nitride substrate of large surface area and complicated shape an oxidation-resistance protective film exhibiting desirable adhesiveness to the silicon nitride substrate and possessing a thermal expansion coefficient approximating that of silicon nitride.